For those without a green thumb, it takes several things to make plants (or algae grow). These are sunlight, nutrients, and water. In the middle of the ocean there is plenty of water and on the surface plenty of sunlight, the problem is lack of nutrients. For almost three decades, oceanographers have been puzzled by the ability of microscopic algae to grow in mid-ocean areas where there is very little nitrate, an essential algal nutrient. In a recent issue of Nature, oceanographer Ken Johnson, along with coauthors Stephen Riser at the University of Washington and David Karl at the University of Hawaii, show that mid-ocean algae obtain nitrate from deep water, as much as 800 feet below the surface.

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Surface waters in the mid-ocean areas contain almost no nitrate or other plant nutrients. In that way it is a sort of desert. Yet microscopic algae (phytoplankton) flourish in these vast, open 0cean areas far from the more nutrioent rich continental shelves.

Although tiny in size, these mid ocean algae consume about one fifth of all the carbon dioxide taken up by plants and algae worldwide. This is the photosynthesis process that takes sunlight and carbon dioxide and converts this into organic compounds. Nitrates are vital too.

Nitrogen is considered a macro element and is a part of amino acids, proteins, coenzymes, nucleic acids and chlorophyll. It is integral to healthy growing algae. When there is not enough Nitrogen, algae will turn yellow at first before either dying or just not growing anymore.

To solve this mystery, Johnson and his fellow researchers used a robotic drifter called an Apex float, which automatically moves from the sea surface down to 3,000 feet and then back again, collecting data as it goes. Researchers at the University of Washington outfitted this drifter with an oxygen sensor and a custom version of Johnson's In Situ Ultraviolet Spectrophotometer, which measures nitrate concentrations in seawater.

In December 2007, researchers from University of Hawaii placed the drifter in the ocean northeast of Oahu, where it collected ocean profiles once every five days for almost two years.

From January through October of each year, the instruments on the drifter showed a gradual increase in oxygen concentrations in the upper 300 feet of the ocean. At the same time, the float detected a gradual decrease in concentrations of nitrate in deeper waters, from 300 to 800 feet below the surface.

Johnson and his coauthors found that the amount of oxygen being produced near the surface through photosynthesis was directly proportional to the amount of nitrate that was being consumed in deeper water.

Because there is not enough sunlight for algae to grow below 300 feet the researchers conclude that algae growing near the surface somehow obtain nitrate from deeper water, and use this nitrate to grow and reproduce. But exactly how the algae obtain these deep nutrients is unclear.

One possible mechanism is the gradual movement of nitrates by ocean eddies. Satellite and drifter data suggest that slow, swirling eddies occasionally form hundreds of feet below the surface of the Pacific. The data suggests that some of these eddies can carry nitrate up to about 200 feet below the ocean surface. Yet these pulses of nitrate do not appear to reach the upper 150 feet of the water surface, where most of the algae grow.

Johnson and his coauthors speculate that dormant algae may inhabit the waters below 300 feet. Open ocean eddies occasionally carry these algae upward, to depths of perhaps 200 feet. At this point, the algae may consume any available nitrate and then migrate farther up into the sunlit surface waters.

Algae are simple celled plants but unexpectedly at first these algae can move to some degree and not just by ocean currents or eddies. Some algae have tiny, whip like flagella that can be used to swim with. Other algae can actively change their buoyancy, just like the Apex float, and either sink or float upwards.

Future research will further evaluate the mechanisms of how the nitrate is moved or where else the required nitrates come from. So the mystery will continue for awhile.

Such studies of tiny algae in the open ocean may seem remote from human activities on land. Yet the oxygen produced by mid ocean algae is essential for the survival of life on earth. Furthermore, these algae move huge amounts of carbon dioxide from the atmosphere into the biological growth, and thus play a significant role in controlling the earth's climate. If they stop doing what they are doing, the effects on the earth's climate will be enormous.

For further information: http://www.mbari.org/news/news_releases/2010/johnson-gyre/johnson-gyre-release.html